In recent years, a fuel cell system has been in the limelight as a next-generation power source system that produces electric energy efficiently and cleanly, and in automobile market and in market of electric power co-generation system typified by a household fuel cell power generation system, field tests for practical implementation aiming at cost reduction have been already executed energetically.
Besides, it has been examined recently to miniaturize the fuel cell system and use as a power source of mobile equipment such as a mobile phone, a PDA (Personal Digital Assistant), a notebook computer, a digital video camera and a digital still camera.
Generally, in a fuel cell, power generation is performed in a manner that hydrocarbon gas such as methane or natural gas (CNG) or alcohol such as methanol or ethanol is used as fuel which is then reformed to hydrogen gas and other gas in a fuel reforming apparatus using a fuel reformer, and thereafter the hydrogen gas is supplied to a power generation apparatus referred to as a power generation cell.
In this case, the fuel reforming in the fuel reformer indicates a process of generating hydrogen gas by a catalytic reaction.
For example, in the case of using methanol as fuel, there are some reactions for reforming the fuel, one of which indicates, for example, a process of generating hydrogen gas (H2) by a steam-reforming reaction as expressed by the following chemical reaction formula (1) (a reaction of bonding steam to methanol and thereby, reforming methanol to hydrogen and carbon dioxide in the formula (1)). Note that a minute amount of generated gas (mainly CO2) other than hydrogen generated by the reforming reaction is usually discharged into the air.CH3OH+H2O →3H2+CO2  (1)
Since the steam-reforming reaction as described above is an endothermic reaction, it is necessary to provide a heater or the like outside for heating the fuel reformer so that a reaction temperature is maintained. Therefore, for reforming the fuel in the fuel reformer, in order to prevent a steam-reforming activity of catalyst from lowering and keep the density of produced hydrogen gas high, a temperature of approximately 200 to 500° C. is required in the case of using methanol as fuel, and a high temperature around 300 to 800° C. is required in the case of using methane gas, for example.
Further, in a partial oxidation-reforming reaction as expressed by the following chemical reaction formula (2), for example, a reforming temperature around 400 to 600° C. is required.CH3OH+1/2O2+2N2→2H2+CO2+2N2  (2)
In view of the foregoing, in the cogeneration power generation system typified by the household fuel cell system, considering that the system itself is large, an outer wall of the fuel reformer housing container is formed into a double-layered structure to thus constitute a vacuum container, or, alternatively, a gap between an inner wall and an outer wall of the double-layered structure is filled up with insulation, to thereby prevent heat inside the fuel reformer from being conducted to outside so that the temperature of the fuel reformer will not decrease. Accordingly, upon housing the fuel reformer into the fuel reformer housing container, it is possible to join the fuel reformer directly to the inner wall of the double-layered structure of the fuel reformer housing container so that the fuel reformer can be placed and fixed in the fuel reformer housing container.
As a related art, there is Japanese Unexamined Patent Publication JP-A 2003-2602.
In recent years, a fuel cell system for use in a mobile equipment has been demanded to be smaller in size and lower in height in order to be housed in the mobile equipment. However, the conventional outer wall of the fuel reformer housing container formed into the double-layered structure is not adoptable in the fuel cell system for use in the mobile equipment because the fuel cell system becomes complicated and larger in size overall. For the fuel cell system for use in the mobile equipment, there has been thus proposed a fuel cell system with a fuel reformer housing container composed of a base having a concave portion and a lid body, of which inner part is formed in a vacuum state to cut off the outward conduction of heat generated in reforming the fuel inside the fuel reformer, resulting in small power generation loss.
In order to use the fuel cell system as described above stably and safely for a long period of time, the vacuum state inside the fuel reformer housing container needs to be maintained not only for a short period just after sealing the fuel reformer housing container in which the fuel reformer is housed, but also for a long period thereafter. However, after sealing the fuel reformer housing container by the lid body, the gas adsorbed to an inner surface of the fuel reformer housing container and surfaces of various components inside the fuel reformer housing container such as a surface of the fuel reformer itself may possibly be released as outgas inside the fuel reformer housing container by influences of a temperature in reforming the fuel and as time passes.
In this case, the level of vacuum inside the fuel reformer housing container drops, thus leading an increase in an amount of outward conduction of heat generated in reforming the fuel inside the fuel reformer, with the result that a temperature of the fuel reformer housing container rises by the heat, which may break other components inside the mobile equipment.
Further, in the case where the fuel-reforming reaction is an endothermic reaction such as the steam-reforming reaction as expressed by the chemical reaction formula (1), the fuel reforming in the fuel reformer requires a heater or the like for heating the fuel reformer so that a reaction temperature is maintained at a constant level, but the heat generated in the fuel reformer is conducted to the fuel reformer housing container as described above, thus causing the temperature of the fuel reformer to be more easily decreased.
In view of the foregoing, the amount of power generation of the heater needs to be increased in order to maintain the reaction temperature, but the increase in the power generation of the heater leads an increase in electric capacity which is used for heating the heater, relative to the total electric capacity generated in the power generation cell of the fuel cell, resulting in a problem that the power generation loss in the entire fuel cell system increases.